crosperf: set recommended iterations for benchmarks

This CL associates estimated standard deviations to each benchmark,
according to experiments. The recommended iterations can be specified by
setting iterations = 0 in the experiment files. Setting it to numbers
greater than 0 will override the default iterations and behaves exactly
the same as before.

With this change, benchmarks in all_toolchain_perf get no more than 2%
margin of error within 90% of time. See crbug.com/673558 for how the
standard deviations are estimated.

BUG=chromium:673558
TEST=all_toolchain_perf + page_cycler_v2.typical_25 finishes in 3.5
     hours for an image on chell.

Change-Id: Ie2ed07878c1237ad31a8568ae3fd3fb96cd11f3b
Reviewed-on: https://chromium-review.googlesource.com/424915


Commit-Ready: Ting-Yuan Huang <laszio@chromium.org>
Tested-by: Ting-Yuan Huang <laszio@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>